Radio frequency (RF) circuits include a variety of structures to transmit RF signals, for example, waveguides, resonant cavities, filters and the like. The RF circuit structures are used for transmitting signals at various frequencies of the electromagnetic spectrum, i.e., radio frequencies, radar frequencies and optical frequencies, by confining and guiding electromagnetic waves through the RF circuit. For example, conventional waveguides may include parallel plates or may be any rectangular or circular cross-sectional pipe structure that confines and guides electromagnetic waves between the first and second locations. As the frequency of the RF signal passed through the RF circuit increases, the size of the circuit structures required to guide and pass the signal decreases. For example, the required width of a waveguide may be as small as a few millimeters depending on the signal frequency, and may even be less than one millimeter (1 mm) to pass signals with extremely high frequencies. For signal frequencies approaching 80 GHz and beyond, circuit structure sizes fall into the micromachining regime. Fabricating micromachined circuit structures to accommodate these high frequency signals is difficult.
One known method for forming circuit structures is through high-precision computer numerical controlled (CNC) machining of metals. CNC machining of metals allows for the fabrication of high aspect ratio structures suitable for RF waveguides. However, manufacturing complexity of CNC machined circuit structures increases as structure size decreases due to the tight specifications required to form the necessary structure geometry, i.e. the hollow cavity of a resonant cavity. This increased complexity results in high relative manufacturing costs and reduces manufacturing yield.
Photolithography is a known technique for microfabrication, i.e. micromachining, of features within a thin film. Lithographic techniques are capable of producing features on a sub-micron to millimeter scale with high precision. However, while effective for formation of features of a thin film, photolithography does not allow for fabrication of the high aspect ratio structures necessary for RF circuit structures because the thin film of the lithographic process must be formed on a flat surface. Once the features are formed in the thin film, the topography of the features prevents further photolithographic layers from being formed thereon.
Therefore, there is a need to provide RF circuit structures for high frequency RF signals that overcome the manufacturing and cost deficiencies of the prior art.